Magnetic resonance imaging (MRI)is a powerful non-invasive tool with high spatial resolution for clinical diagnosis without the limitation of the depth of tissues. A major barrier to the application of MRI technique is its lack of sensitivity and specificity. The method routinely uses contrast agents to amplify the contrast in the magnetic resonance image between pathological regions and normal tissues. Although remarkable progress in the development of contrast agents with improved properties has been made in the last twenty years, there is still a great need for the development of contrast agents with even higher contrast capability in imaging and to target to specific molecular entities. The goal of this project is to develop a novel class of protein-based MRI contrast agents with significantly improved contrast capability for different organs and applicability in molecular imaging of various types of cancer and other diseases. In this proposal, Aim 1 is to develop the protein contrast agents with high relaxivity. Aim 2 is to test our designed agents in in vivo imaging and determine the in vivo relaxivity. In addition, LD50 and maximal tolerable doses will be determined using mice. Further, to reduce the immunogenicity, we will modify protein contrast agents by PEGylation. The immunogenesity, blood circular time, stability and pharmacology of the modified proteins will be investigated. Aim 3 is to test the feasibility of applying our developed contrast agents for molecular imaging of disease biomarkers with cultured cancer cells. Our proposed work explores a new mechanism to overcome a major limitation of low sensitivity and selectivity of contrast agents facilitating diagnosis and monitoring treatment of various diseases byMRI.